During assembly of a commercial aircraft, certain operations are performed synchronously inside and outside of confined spaces. Consider a fastening operation on a spar to panel of a wing box. An assembly system outside the wing box performs drilling, countersinking and fastener insertion tasks on one side of the panel. A person or system inside the wing box on an opposite side of the panel places a sleeve and nut over the inserted fastener.
During drilling, the assembly system needs to apply a proper clamping force against the panel. If an electromagnet is being used to apply the clamping force, the clamping force may be estimated according to current supplied to the electromagnet's coil. However, the estimation is not always correct. An incorrect estimation can lead to insufficient clamping force.
If the clamping force is insufficient during a drilling operation, the drill can shift. Consequently, the location of the drilled hole could be out of tolerance. In view of then number of holes that need to be drilled with precision in a commercial aircraft, the cost of out-of-tolerance holes can be quite substantial.
It would be desirable to accurately measure the clamping force during drilling instead of estimating the clamping force.